The "Swinging Flashlight Test:" What Your Eye Doctor is Looking For

The Marcus Gunn Sign is a measurement of how the pupils in the eye react to a swinging light. When a bright light is swung from one eye to another, the pupil of the affected eye will constrict less. The Marcus Gunn Signs is also known as Relative Afferent Pupillary Defect.

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What is supposed to happen?

In an individual who does not suffer from the Relative Afferent Pupillary Defect (or Marcus Gunn pupil), shining a light in one eye or the other should not affect the expansion or constriction of the pupil in either eye – the eyes should react as one. When the light is shone directly into the affected eye, the pupil in this eye will constrict less than it would when the light is shone into the unaffected eye.

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What does the "swinging light test" indicate?

The Marcus Gunn sign often indicates the presence of a lesion on the optic nerve. Testing for RAPD is a good way to implicate or rule out optic nerve damage such as is caused by optic neuritis, a common symptom of MS.

Damage to the optic nerve is the most common cause and damage to the optic nerve is very common in multiple sclerosis. But that does not mean that anybody who shows this sign has or might have MS. If you are concerned about any unusual symptoms or signs you notice, then contact your doctor to discuss the possible causes and treatments recommended.

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What other tests should be conducted to see if I have MS?

A patient may first experience changes in vision, vertigo, weakness or numbness in the limbs, balance problems or difficulty walking, among others. If experiencing any of these symptoms, a patient should conduct a doctor. From there, the doctor can administer a variety of tests can be used to diagnose MS including a CAT scan, MRI, lumbar puncture, EEG or EMG/NCV.